Nd and Sr isotopic ratios and trace element geochemistry of epidote from the Swiss Molasse Basin as provenance indicators: implications for the reconstruction of the exhumation history of the Central Alps

Epidote is a common heavy mineral in sandstones. Its use as a provenance indicator, however, is limited because it appears in a wide range of lithologies and geological settings. In order to discriminate between mantle- and crust-derived source rocks, we measured isotopic and trace element compositions of detrital epidote from sandstones of the Swiss Molasse Basin. The compositions of the detrital epidotes were compared to epidotes from Penninic metabasites and Austroalpine metagranites from the present-day exposures and to epidote from ophiolitic and metagranitic pebbles from the molasse basin. We found that Nd isotopic ratios of epidote allow a good discrimination between different types of source rocks with εNd values ranging between 6.1 and 8.6 for metabasic lithologies and between −9.7 and −2.8 for metagranitic lithologies. Sr isotope ratios reveal similar trends but do not allow such a clear discrimination. Ba, U, Rb, Nb and Th contents of epidote correlate well with the εNd values, yielding the same provenance signal. ing to isotopic and trace element compositions, three types of epidote-bearing sandstones can be distinguished. One received epidote from Lower Austroalpine metagranites, another from Penninic ophiolites and a third type of sandstone received epidote from both sources. The provenance of the detrital epidote has important bearing on the exhumation history of the Central Alps, elucidating the transition between the erosion of Lower Austroalpine felsic lithologies and the onset of erosion of the Penninic lower plate. According to our data, the Penninic lower plate became exposed simultaneously in the hinterland of all molasse fans at 21–20 Ma. At the same time, the relief collapsed, as reflected by a drastic decrease of sediment accumulation rates in the foreland basin. The combination of both events fits well to the geodynamic scenario of large-scale lateral extension processes affecting the Central Alps in post-collisional times.